A conventional speaker system will be described first, with reference to FIG. 5. FIG. 5 is a sectional view of a conventional speaker system. In the structure shown in FIG. 5, magnet 1 is placed between upper plate 2 and yoke 3 to form inner magnetism-type magnetic circuit 4. Yoke 3 of magnetic circuit 4 is connected with frame 6. On the periphery of frame 6, first diaphragm 7 is attached and further on which, voice coil 8 is fixed to fit in magnetic gap 5 of magnetic circuit 4.
Besides, panel 9 is fixed on the periphery of frame 6, and further on panel 9, substantially flat second diaphragm 10 is attached. With the structure above, first diaphragm 7 and second diaphragm 10 are acoustically coupled through enclosed room 11. Sound through-hole 12 in enclosed room 11 acoustically couples first diaphragm 7 to second diaphragm 10, determining the positional relation between the two diaphragms. A speaker system having such a through-hole is introduced, for example, in Japanese Patent Unexamined Publication No. 2003-179988.
The conventional speaker system, however, has inconveniencies—second diaphragm 10 has often suffered thermal deformation due to high temperatures, or scratches on the surface by rubbing against other objects because of being exposed.
Furthermore, second diaphragm 10 can warp by its own weight, or can be pulled to panel 9 by static electricity, so that panel 9 has often made a tight contact with second diaphragm 10 and has hampered vibrations of diaphragm 10. This has often increased sound distortion, which can be a threat against improvements in performance and reliability of the speaker system.